1. Field of the Invention
The invention relates to an electric resistance melting furnace for vitrifiable compositions, such as glass, enamel or ceramics, with a rotatable melting tank and a stationary upper furnace, which can be operated intermittently or continuously with a "cold top" cover and permits a rapid and clean product change.
2. Description of the Related Art
Various types of heating furnace which differ in, inter alia, the form of energy which is used are employed in the glass, enamel and ceramic industries for melting vitrifiable compositions. For example, gas-heated or electrically heated furnaces are used in parallel, the economic efficiency of these in each case being determined by their energy consumption, by the quantity and composition of their waste gases and by their flexibility with regard to a product change. The known furnaces have until now only met selected criteria relating to a narrow circle of products to be treated.
There is, however, interest in a type of furnace which permits a frequent change of product, while keeping losses in terms of raw material to a minimum, yet which also permits flexible operating conditions which in turn enable a wide range of products to be processed.
Generally speaking, the types of melting furnace used for vitrifying raw materials are distinguished according to the type of energy supply. For example, furnaces with radiant heating through flames or electric radiant elements have become known, as have those which operate by direct heating on a reverberatory basis (see, for example, EP 71 110).
Furnaces as described in EP 71 110 use a batch layer at the lining in order to prevent the latter from sustaining damage due to the heat of the flames.
Since it is impossible to prevent damage to the lining in the above-mentioned types of furnace due to gaps in the outer batch layer and the thermal load, in spite of being vertically rotatable, specific expenditure is additionally employed for separate cooling or a refractory lining in recent developments (see, e.g. EP 231 516 B1).
Those electrically heated melting furnaces which have resistance heating, flame arc heating or induction heating have in particular gained importance. An electric resistance melting furnace has been described, for example, in the patent specification DE 38 24 829. Electric resistance furnaces and similar furnace types can be operated with an open melting bath or even with a melting bath covered with solid raw materials, a so-called "cold top" cover. The melting tanks which are covered with raw material require a complex raw material charging machine in order to ensure that the raw material cover is composed in a uniform manner.
The known electric resistance melting furnaces have a stationary tank with fixed heating electrodes and are optimised in terms of structure and geometry in line with a certain product. Furnaces of this kind are usually conceived for long-term, continuous operation without a product change-over.